Zinc oxide (ZnO) having bandgap energy of about 3.37 eV and exciton binding energy as large as 60 meV is considered promising in industrial applications, particularly in an application to self-emitting electronics devices, as a phosphor or light emitting material of near-ultraviolet light (wavelength of about 390 nm) that emits light stably even at room temperature.
A phenomenon has been known generally that, when the size of semiconductor is reduced to be comparable to the wavelength of electron or hole wave function in the semiconductor, so-called quantum confining levels appear in conduction band and valence band and the electrons and holes increase ground state energy. In other words, when a nano-sized semiconductor is formed, the bandgap of the semiconductor becomes larger due to quantum size effect, and emission energy due to recombination between the excited electron in conduction band and the hole in valence band increases as the semiconductor size decreases. Attempts have been made to produce material (a so-called quantum dot material) that can provide near-ultraviolet emission having desired emission peak wavelength and desired half width, utilizing the quantum size effect, by controlling the size of zinc oxide nano particles.